This invention relates to the art of multi-zone reheat furnaces and, more particularly, to a method and control system for achieving energy conservation in connection with the use of such reheat furnaces.
The reheating of workpieces such as steel billets, blooms or slabs is often achieved in a multi-zone reheat furnace having sequentially arranged preheating, heating and soaking zones through which the workpieces are advanced to heat the workpieces to a desired process temperature appropriate for working the workpieces, such as for example by hot rolling or forging. It is of course well known that the preheating and heating zones of such furnaces are fired to achieve heating of the workpieces passing therethrough at a given throughput rate so that the workpieces are elevated to the soaking temperature when they enter the soaking zone. With respect to the given throughput rate, it is of course desirable to achieve such heating of the workpieces with minimum fuel consumption in connection with the firing of the preheating and heating zones. At the same time, it is important that the workpieces reach the soaking temperature no later than the time of entrance into the soaking zone to assure that the workpieces are thoroughly heated when they are discharged from the furnace. In this respect, the process temperature is a critical factor in connection with obtaining an acceptable product when a workpiece is rolled, forged or otherwise worked following discharge from the furnace. Accordingly, it has been the practice heretofore with either manual or automatic control systems for such reheat furnaces to heat the workpieces during movement thereof through the preheating and heating zones so that the soaking temperature is reached early in the heating zone and is substantially reached in the preheating zone. As a result of such practice, the workpieces are held at or near the process temperature for longer periods than desired, and this causes the production of excessive scale or metallic oxides which can result in the loss of a saleable end product. Furthermore, such quick elevating of the workpiece temperature requires a high temperature profile across the preheating zone and, accordingly, excessive consumption of fuel in connection with firing the latter and excessive thermal loss as a result of high waste or exhaust gas temperatures.
Manual control of the temperatures in the preheating and heating zones of a reheat furnace, to be effective, requires operator experience together with a high degree of attentiveness to conditions within the furnace. In the absence of such experience and attentiveness, the work in the furnace can be severely damaged. In any event, manual control based on existing furnace or workpiece operating or condition characteristics is not efficient with respect to fuel consumption. In this respect, the operator is often personally responsible for the condition of the workpieces exiting from the furnace and, accordingly, will maintain higher than necessary temperatures in the furnace to assure thorough heating of the workpieces when they exit from the furnace. A variety of automatic control systems having been devised heretofore in an effort to increase the efficiency in operation of reheat furnaces with respect to fuel consumption and product quality, but these systems, as a result of instrumentation used and/or the furnace or workpiece conditions to which they are designed to respond, do not enable achieving a temperature profile across the preheating and heating zones which minimizes fuel consumption and thermal losses while assuring that workpieces are maintained in the furnace at the process temperature for a minimum amount of time during continuous furnace operation. Furthermore, such previous systems are slow in response to either an increase or a decrease in throughput rate, thus limiting the operating efficiency of the system and, additionally, the ability to achieve smoothness and accuracy with respect to varying burner operating functions in connection with varying throughput rates.